In humans, such traits as left- and right-handedness
are enduring characteristics of individuals, and it is precisely their stability
that accounts for their central place in theories pertaining to lateral cerebral
dominance. Similar stability of left-right behaviors in marine mammals would
likewise carry implications about cerebral asymmetry in these species. For this
reason, we tested for consistent left/right motor biases in captive California
Sea Lions. We assessed lateral bias in two settings—in foot-fall patterns when
running and in direction of circling when swimming.

Procedure

Thirty two adult California Sea Lions (14 female, 18 male), housed at five
different Seaquariums, served as subjects. We made observations on each of two
behaviors and all subjects were observed at least two times on each measure.
For half of the subjects, we completed at least one observation per measure each
week for ten weeks.

Circling While Swimming. For a one minute period, the
observer noted the number of complete circuits that each Sea Lion made in a
counterclockwise direction (scored as leftward) or clockwise direction (scored
as rightward) in its holding pool.

Running Gait.
Each Sea Lion was videotaped while running, once directly toward, and once
directly away from, the camera. The leading front flipper and asymmetrically
swung hind flipper were noted in subsequent video analysis. Separate ratings by
two independent blind observers achieved very high inter-rater reliability on
these observations.

For
each measures, a numerical score was computed as the number of rights divided by
the total number of lefts plus rights (R/(L+R)). For comparisons across
behaviors, the mean score across observations was computed separately for
running and swimming.

Results

High within measure reliability.

Both
measures were characterized by very high reliability across observations-both
within and across days (r = 0.7+, p < .0001). That is, individual Sea Lions had
distinct preferences to favor their left or right flipper leads and to prefer
left or right circling in a way that was very consistent within animals over
time.

Lack of population bias.

Across the sample, there was no significant directional bias on either task—that
is, on both measures there were approximately equal numbers of right- and
left-biased animals. Neither were there reliable differences between the sexes
on overall direction of bias.

Low between measure correspondence.

Across all subjects, the correlation between the two forms of locomotion (mean
lateral bias across observations on each measure) was 0.24 (ns).

Sex differences.

Although, the reliability across observations was high and statistically
significant in both sexes, there were differences between them that approached
(but did not reach) statistical significance. Specifically, there was greater
consistency in running flipper lead in females (r = 0.8 to 0.99) than in males
(r = 0.5 to 0.7). And, by contrast, there was greater consistency in direction
of swimming in males (r = 0. 7 to 0.8) than in females (r = 0.5 to 0.6).
Moreover, the inter-measure correlation was higher in females (r = 0.37) than in
males (r = 0.11).

Discussion

The
very high day-to-day reliability for the lateral bias shown in each form of
locomotion implies that individual Sea Lions are characterized by enduring
asymmetries in the neural substrate that underlie their locomotion. However,
this asymmetry appears to vary from animal to animal and does not show the type
of population-wide bias evidenced by the predominant right-handedness of
humans.

Moreover, the lack of concordance between the biases shown in running and
swimming suggests the absence of any consonant cerebral asymmetry in this
species. In this regard, the California Sea Lion more closely corresponds to
other non-human mammal species such as the rat (Noonan and Axelrod, 1989),
wherein no directional concordance has been found between lateral biases on land
and while swimming.

The
possible finding of sex differences in the degree of concordance between our
laterality measures is also reminiscent of a somewhat similar pattern in rats
(and in humans). However, we will need to add additional subjects to our
analysis before a confident statement can be made about any real sex differences
in sea lions.

More
importantly, now that we know that individual sea lions possess enduring lateral
biases, it will be interesting in future studies to see if left- and
right-biased animals characteristically differ on measures of cognitive
processing in ways that may be analogous to such differences in humans.

Acknowledgements

The
authors wish to thank Nic Hayne, Dave Elliott and John Holer of Marineland of
Canada; Emily Insalaco, Michelle Grady and Jennifer Humphrey of the Aquarium of
Niagara; Marci Romagnoli, Tim Hoffland, Melanie Rood and Moby Solangi of Marine
Life Productions; Susan DeFalco and Jeff Wyatt of the Seneca Park Zoo; all for
their generous contributions of time, logistical support and the access to
animals that made this investigation possible.